1. Field of the Invention
The present invention relates to a method for controlling fouling in equipment for processing and storing hydrocarbon compositions containing unsaturated compounds. More specifically the processing may include, for example, preheating, hydrogenation, fractionation, extraction and the like of hydrocarbon streams to remove, concentrate, or have added thereto the unsaturated hydrocarbons prior to storage or use.
The process of recovering olefinic hydrocarbons from gas and liquid cracking operations, the conversion of olefinic and acetylenic compounds by, for example hydrogenation and the separation of the various olefins and acetylenic compounds by distillation or extraction is complicated by the formation of fouling deposits on the heat transfer surfaces of the processing equipment. These deposits decrease the thermal efficiency of the equipment and decrease the separation efficiency of the distillation towers. In addition, operating modifications to reduce the rate of fouling can result in reduced production capacity. The excessive build-up of such deposits can cause plugging in tower plates, transfer tubes, and process lines, which could result in unplanned shutdowns. These deposits are generally thought to result from free radical polymerization induced thermally, by contaminating oxygen or by metal ions. Similar problems are also encountered in olefin recovery operations not connected with thermal cracking, for example, in styrene recovery.
Such fouling can be controlled according to the present invention, by the judicious use of an appropriate stable free radical or a suitable precursor that under the process conditions yields the active stable free radical in situ. The fouling is controlled by the action of the stable free radicals in terminating radical polymerization chain reactions in the processing equipment.
2. Related Art
The use of stable nitroxides and other stable free radicals and precursors thereto are well documented in the patent and open literature as stabilizers for olefinic organic compounds. The prior art teaches that these stable free radicals are useful for the prevention of premature radically induced polymerization of the olefinic monomer during storage and as antioxidants. Typical of this art is U.S. Pat. No. 3,747,988 on the stabilization of acrylonitrile; U.S. Pat. No. 3,733,326 disclosing stabilization of vinyl monomers by free radical precursors; U.S. Pat. No. 3,488,338 on the stabilization of chloroprene; UK Pat. Spec. No. 1,127,127 relating to the stabilization of acrylic acid; UK Pat. Spec. No. 1,218,456 relating to the stabilization of butadiene and the following publications "Inhibition of Radical Polymerization by Nitroxide Mono and Biradicals", L. V. Ruban, et al, Vysokomol. soyed. 8: No. 9, p. 1642-1646, 1966; "Iminoxy Radicals as Inhibitors of w-Polymerization of Chloroprene", M. B. Nieman, et al, Vysolomol. soyed, 8: No. 7, p. 1237-1239, 1966; "Inhibition of Polymerization of Styrene by a Stable Radical with 4,4'diethoxy-diphenyl Nitric Oxide", M. D. Goldfein, et al, Vysokomol. soyed. A 16: No. 3, p. 672-676, 1974; "Inhibition of Polymerization of Vinyl Monomers by Nitroxide and Iminoxyl Radicals", A. V. Trubnikov, et al., Vysokomol. soyed. A20: No. 11, p. 2448-2454, 1978.
This art, without exception discloses the use of stable free radicals in excess of 1 ppm and generally well in excess of 10 ppm. Although there is a body of art relating to stable free radicals, these materials are very expensive and usually available or produced only in very limited quantities for research. Hence, the art has consistently taught that which it considered to be the smallest effective amount of stable free radical for stabilization as at least 1 ppm (actually the major portion of the art teaches over 10 times this amount). It has now been surprisingly found that fouling caused by polymerization of olefinic organic compounds can be effectively controlled with substantially less than the 1 ppm of stable free radical taught by the art as the lowest level. Although the stable nitroxide free radical compounds are currently, principally of academic interest there is an abundance of art on their preparation, for example, the following U.S. Pat. Nos., 3,494,930; 3,966,711; 3,704,233; 3,334,103; 3,253,015; 3,372,182; 3,502,692; 3,422,144; 3,163,677 and 3,873,564.
It is an advantage of the present invention that fouling in processing equipment for hydrocarbon streams containing olefinic organic compounds, may be controlled by the present invention. It is a feature of the present invention that stable free radicals, which have been laboratory curiosities until now because of their high cost, can now be beneficially employed commercially for stabilization. These and other advantages and features will become apparent from the following description.